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ABOUT THIS IMAGE:

Looking like a child's pinwheel ready to be set a spinning by
a gentle breeze, this dramatic spiral galaxy is one of the
latest viewed by NASA's Hubble Space Telescope. Stunning
details of the face-on spiral galaxy, cataloged as NGC 1309,
are captured in this color image.

Recent observations of the galaxy taken in visible and
infrared light come together in a colorful depiction of many
of the galaxy's features. Bright blue areas of star formation
pepper the spiral arms, while ruddy dust lanes follow the
spiral structure into a yellowish central nucleus of older-population stars. The image is complemented by myriad far-off
background galaxies.

However, this galaxy image is more than just a pretty
picture. It is helping astronomers to more accurately measure
the expansion rate of the universe. NGC 1309 was home to
supernova SN 2002fk, whose light reached Earth in September
2002. This supernova event, known as a Type Ia, resulted from
a white dwarf star accreting matter from its companion in a
binary star system. When the white dwarf collected enough
mass and was no longer able to support itself, the star
detonated, becoming the brightest object in the galaxy for
several weeks.

Nearby Type Ia supernovae like SN 2002fk in NGC 1309 are used
by astronomers to calibrate distance measures in the
universe. By comparing nearby Type Ia supernovae to more
distant ones, they can determine not only that the universe
is expanding, but that this expansion is accelerating.
However, this method only works if the distance to the host
galaxies is known extremely well.

That's where the Hubble Telescope comes into play. Since NGC
1309 is relatively close to us, the high resolution of
Hubble's Advanced Camera for Surveys can help accurately
determine the distance to the galaxy by looking at the light
output of a particular type of variable star called a Cepheid
variable. Cepheids are well studied in our own galaxy, and vary
regularly in brightness at a rate that is directly related to
their total intrinsic brightness. By comparing their variation
rate with how bright they appear, astronomers can deduce their
distance. In this way, the Cepheids in NGC 1309 allow astronomers
to accurately measure the distance to NGC 1309, and thus to
SN 2002fk. The expansion of the universe was discovered by
Edwin Hubble, the Hubble Space Telescope's namesake, nearly
a century ago, but the accelerating expansion is a recent
discovery which has interesting consequences for cosmological
models.

These Hubble images were taken in August and September 2005.
NGC 1309 resides 100 million light-years (30 Megaparsecs)
from Earth. It is one of about 200 galaxies that make up the
Eridanus group of galaxies.